The present invention is directed to a manhole rim and cover assembly which, while useful in other applications, is particularly well-adapted for use in manholes utilized in gasoline service stations to provide access to underground storage tank fill pipes.
In the past, fill pipe manholes for underground gasoline storage tanks consisted simply of concrete or steel sidewalls closed at their upper end by a cover with the bottom of the manhole being soil. Gasoline spilled into the manhole when the supply hose from the tank truck was uncoupled from the fill pipe simply drained into the ground. Present-day environmental concerns find most fill pipe manholes now incorporating an overfill storage container such as that disclosed U.S. Pat. No. 4,793,387. Such containers typically consist simply of a bucket-like reservoir mounted on the fill pipe below its upper end to capture fuel which might be spilled when the supply hose is uncoupled. A drain valve is usually provided to drain captured fuel from the vessel back into the fill pipe when the level of fuel drops sufficiently.
It is believed apparent that in such a spill containment device application, the manhole cover should be sealed sufficiently tightly to the rim so that water cannot leak into the reservoir to be subsequently drained into the storage tank along with overfill fuel. However, if a water-tight seal between cover and rim is employed, temperature variations under some circumstances can produce a partial vacuum within the reservoir which will make removal of the cover extremely difficult.
Insofar as retaining the cover in position upon the manhole rim, the prior art typically either simply relies upon gravity to hold the cover in position or alternatively utilizes bolts. Reliance on gravity alone is sometimes insufficient, while the utilization of bolts requires that the bolts be removed to release the cover and replaced to reseal it. In order to replace the bolts, it is necessary to precisely align the bolt holes the cover with the bolt hole receiving bores in the rim.
The present invention is directed to a manhole cover and rim assembly which addresses the problems referred to above.